Deep brain stimulation (DBS) refers to the delivery of electrical pulses into one or several specific sites within the brain of a patient to treat various neurological disorders. For example, deep brain stimulation has been proposed as a clinical technique for treatment of chronic pain, essential tremor, Parkinson's disease (PD), dystonia, epilepsy, depression, obsessive-compulsive disorder, and other disorders.
A deep brain stimulation procedure typically involves first obtaining preoperative images of the patient's brain (e.g., using computer tomography (CT) or magnetic resonance imaging (MRI)). Using the preoperative images, the neurosurgeon can select a target region within the brain, an entry point on the patient's skull, and a desired trajectory between the entry point and the target region. In the operating room, the patient is immobilized and the patient's actual physical position is registered with a computer-controlled navigation system. The physician marks the entry point on the patient's skull and drills a burr hole at that location. Stereotactic instrumentation and trajectory guide devices are employed to control the trajectory and positioning of a stimulation lead during the surgical procedure in coordination with the navigation system.
The proximal end of the stimulation lead is tunneled underneath the skin of the patient. Often, the terminals of the stimulation lead are coupled to an “extension” lead. The extension lead is also tunneled for connection to an implantable pulse generator (IPG). The IPG is usually implanted within a subcutaneous pocket created under the skin by a physician. The IPG generates the electrical pulses for the patient therapy. The electrical pulses generated by the IPG are provided through the feedthroughs and header electrical connectors of the IPG through the extension lead to the terminals of the stimulation lead, through the wire conductors, and eventually to patient tissue through the electrodes.
There are concerns related to the compatibility of deep brain stimulation systems and other stimulation systems with magnetic resonance imaging (MRI). MRI generates cross-sectional images of the human body by using nuclear magnetic resonance (NMR). The MRI process begins with positioning the patient in a strong, uniform magnetic field. The uniform magnetic field polarizes the nuclear magnetic moments of atomic nuclei by forcing their spins into one of two possible orientations. Then an appropriately polarized pulsed RF field, applied at a resonant frequency (about 64 and 128 MHz for 1.5 T and 3.0 T MRI systems, respectively), forces spin transitions between the two orientations. Energy is imparted into the nuclei during the spin transitions. The imparted energy is radiated from the nuclei as the nuclei “relax” to their previous magnetic state. The radiated energy is received by a receiving coil and processed to determine the characteristics of the tissue from which the radiated energy originated to generate the intra-body images.
Currently, deep brain stimulation systems are designated as being contraindicated for MRI, because the time-varying magnetic RF field causes the induction of current which, in turn, can cause significant heating of patient tissue due to the presence of metal in various system components. The heating of patient tissue can cause cell necrosis. Depending upon the implant location of the electrodes of the stimulation lead, heating of the brain tissue can result in significant neurological impairment and even patient death.
The current induced by an MRI system through a stimulation lead can be “eddy current” and/or current caused by the “antenna effect.” As used herein, the phrase “MRI-induced current” refers to eddy current, current caused by the antenna effect, and/or any other current generated by the time-varying fields of an MRI-system.
“Eddy current” refers to current caused by the change in magnetic flux due to the time-varying RF magnetic field across an area bounding conductive material (i.e., patient tissue). The time-varying magnetic RF field induces current within the tissue of a patient that flows in closed-paths. When a conventional pulse generator and a conventional implantable lead are placed within tissue in which eddy currents are present, the implantable lead and the pulse generator provide a low impedance path for the flow of current. Electrodes of the lead provide conductive surfaces that are adjacent to current paths within the tissue of the patient. The electrodes are coupled to the pulse generator through a wire conductor within the implantable lead. The metallic housing (the “can”) of the pulse generator provides a conductive surface in the tissue in which eddy currents are present. Thus, current can flow from the tissue through the electrodes and out the metallic housing of the pulse generator. Because of the low impedance path and the relatively small surface area of each electrode, the current density in the patient tissue adjacent to the electrodes can be relatively high. Accordingly, resistive heating of the tissue adjacent to the electrodes can be high and can cause significant, irreversible tissue damage.
Also, the “antenna effect” can cause current to be induced which can result in undesired heating of tissue. Specifically, depending upon the length of the stimulation lead and its orientation relative to the time-varying magnetic RF field, the wire conductors of the stimulation lead can each function as an antenna and a resonant standing wave can be developed in each wire. A relatively large potential difference can result from the standing wave thereby causing relatively high current density and, hence, heating of tissue adjacent to the electrodes of the stimulation lead.